Fuel cells of the high temperature, solid electrolyte type use thin layers of a solid electrolyte (.about.40 .mu.m), primarily stabilized zirconia, to convert the chemical energy of unrefined fuels, such as carbon monoxide-hydrogen mixtures, into direct current electrical energy at temperatures at or above 800.degree. C. Such temperatures are required to render the solid electrolyte sufficiently conductive to minimize ohmic losses.
Individual fuel cells produce between 0.5 volt and 1.0 volt requiring that many cells be interconnected in both series and parallel to obtain the desired output voltage and current. In addition, proper operation of a fuel cell assembly requires that reactants be separated in the cell in order not to harm the electrodes. Further, the incoming oxidant must be preheated to near operating temperature.
The fuel cell generator described in U.S. Pat. No. 4,395,468 presents a configuration that satisfies the above requirements. Using the method of cell interconnection disclosed in the above cross-referenced application Ser. No. 323,641 individual fuel cells are electrically connected in series and parallel along their lengths to reduce ohmic losses and obtain the desired output voltage and current. Fuel cell reactants are kept separated, not by hermetically sealing between cell electrode compartments, but by controlled pressure differences in such compartments, to allow spent oxidant and fuel to combine in a combustion chamber which also serves to preheat incoming oxidant within air feed tubes that reach into the tubular cells.
Certain drawbacks remain in the design of U.S. Pat. No. 4,395,468. Individual cells at the periphery of the generator cell stack are in contact with restricting walls of insulation or containment. Depending upon the closeness of contact, fuel can leak without undergoing electrochemical oxidation along cell to wall contact areas. Close tolerances, careful sizing, and proper insulation support must be considered to limit fuel leakage to a minimum along the interface.
Also in the U.S. Pat. No. 4,395,468 design electrical contacts are made from cell to cell along the tube axis. Should one cell fail to operate in a single string of cells, one would lose power of the whole string unless more cells are also parallel connected. Another possible drawback of this generator is the requirement that the fuel cells be straight and of close tolerance for reliable electrical connection and that mechanical bundle cohesiveness is determined by the mechanical strength of the electrical contact strip bonds along a string of tubes. As shown in FIG. 1, failures may occur either in the parallel direction 9, series direction 10 or diagonal direction 11 through the tube bundle.
Other arrangements of individual fuel cells have been disclosed in the prior art, but not arrangements of the annular type of cell. U.S. Pat. Nos. 3,717,506, 4,175,165 and 4,248,941 pertain to solid electrolyte fuel cells that comprise large numbers of stacked planar fuel cells. These designs use connected plates rather than annuluses to form the generator array.